Machinist?? Indexing with dividing head

[pacer 3,159]

Many years ago I was into green sand casting and I wanted to make a couple gears from a casting ....so my talented machinist friend taught me how to index with a dividing head, but I now cant go to him for help - unfortunately he passed last year. And, he did a good job because as time passed I cast and perfectly indexed maybe 40-50 gears (one was 121 teeth!) ---

 

So here we are today and indexing this cogged pulley has me ... frustrated?

 

What I have -- the black plastic pulley is off a MF 1855 garden tractors 54" deck which is shaft driven thru a gear box that has a splined center and as you can see that if/when you hit a root/buried stump/etc those gear driven splines will shuck the teeth like an ear of corn! and like many of our horse parts this thing is not to be found. That one is the 3rd one that I have 'shucked" and I cannot find another! So, I thought I would go back in time and try and cast one..... and, after 3 failures, I succeeded. The ... stickler here is the splines! Splines and cutting them never came up with my friend and I have no idea how to cut them so I am using a spare spindle and casting around it (see 2nd pic) and then driving the spindle out of the casting leaving their imprint.... But I quickly found that I could not pull the pattern from the sand without the cogs impression breaking. So I figured I could mill them back by indexing using an 8mm ball end mill. 

 

So, heres where this, somewhat long, post is heading.... There are 41 "cogs" so I need to cut 41 ,,, grooves? into the piece, and using the chart I am using a 41hole plate and turning thru 40 holes to make the next cut (should be able to see that on the chart shown) This works beautifully thru about 12-15 cuts when the cuts start spreading and quickly ruins the piece. This would seem that either the plate or turns arent correct, or both? Could somebody check this for me --- maybe have a different chart or maybe see something I am doing wrong..... I can easily cast another, but till I solve the indexing problem .....

 

This Massy tractor is just a really nice tractor and it PO's me that they used plastic in this manner!!

 

 

Edited August 5, 2021 by pacer

[WHX?? 46,724]

Pics didn't come through for me Pacer anyone else? 

[8ntruck 6,363]

Trying to make an internal spline, right?  In smaller sizes, the spline could probably be broached.  Check Mccmaster Carr to see if they stock a spline broach that matches your application.

 

On the other hand, the splines were probably molded into the original plastic gears.  The gears might have been intended to be a mechanical 'fuse' for when one runs over stumps, roots, rocks, etc.  You might want to ask yourself where the next weakest link is in the drive system - that is what will fail next time you hit a stump, root, rock, etc after you have made a stronger gear.

[pacer 3,159]

I've got the spline problem solved by casting with it in the cast and then driving it out of the cast leaving the splines imprint. The problem is getting the indexing of the cog spacing figured. 

 

I'll try to see about the pics....

 

edit: OK, tried another way to get pics to show, hopefully got it this time

Edited August 5, 2021 by pacer
fix pics

[squonk 38,902]

I still can't see them.

[Achto 26,537]

7 hours ago, pacer said:

edit: I just shut down the forum and rebooted it and the pics came out OK......

 Sorry, still no pics here.

[WHX?? 46,724]

Dan and I got a bro who is heavy into that breed Pacer we'll ask him what he knows. 

[threepiece 6]

I think you may be getting a stack up error. I wonder if your spindle position (radius of tool path) is off a bit for the diameter cutter you are using.

[pacer 3,159]

@threepiece I finally got one made using a .... cheating method?? What I finally came to the conclusion was that what is needed to cut a cog is to have a designated COG cutter, like when cutting gear teeth you have to have the correct one - and there are MANY.

 

I was trying to use an 8mm ball end mill and that was throwing the spacing off - (stacking as you put it). I had 2-3 of the pulleys with the splines ripped out, but the cogged teeth were good, so I took my blank casting and sandwiched it next to the good toothed one and putting aside the 8mm cutter and modified a 1/4" ball mill and with several trips to the grinder made a pretty decent profile of the cogs. I would then line up the cutter with an existing tooth on the plastic pulley and feed it across to the blank casting. By doing this at each tooth I went around and cut all the 41 teeth and it turned out rather nice!

Been using it through 8-10 cuttings of my 1 1/2 acres and its holding up well.......